Combustion in an industrial furnace or combustion in an automobile engine produces nitrogen oxide gas, NOx, which includes nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2). Since it is harmful to humans, it is important to measure the concentration of NOx in the emission gas or in the atmosphere. One of the apparatuses for measuring the NOx concentration is the chemiluminescent analyzer.
A chemiluminescent analyzer for measuring concentration of NO is illustrated using FIG. 1. Sample gas taken from the emission gas or from the atmosphere is mixed with reaction gas including ozone (O.sub.3) in a reaction space of the chemiluminescent analyzer 10, where NO and O.sub.3 chemically react to generate light. The light generated by the reaction is measured by a photometer of the chemiluminescent analyzer 10, whereby the concentration of the NO in the sample gas is calculated. Concentration of ammonia (NH.sub.3) and nitrogen dioxide (NO.sub.2) can be similarly measured by converting them to NO beforehand in a separate reaction chamber.
The reaction space of a conventional chemiluminescent analyzer 10 is detailed in FIG. 7. A double tube composed of an outer tube 51 and an inner tube 52 opens against the photometer 57 (e.g., a photo-multiplier or a photodiode) with a transparent glass plate 56 between them. The end of the inner tube 52 is set withdrawn from the end of the outer tube 51 with respect to the photometer 57. The sample gas 13 coming through the inner tube 52 and the reaction gas 14 coming through the outer tube 51 (precisely saying, between the outer tube 51 and the inner tube 52) meet at a space 55 formed by the wall of the outer tube 51 and the end of the inner tube 52. The NO in the sample gas 13 and the O.sub.3 in the reaction gas 14 react as EQU NO+O.sub.3 .fwdarw.NO.sub.2 +O.sub.2,
and generate light, which is received and measured by the photometer 57.
A problem about the structure is that the sample gas 13 and the reaction gas 14 do not mix well in the reaction space 55 because they come there in the same direction. Therefore the efficiency of the emission of light from the reaction is low, and the linearlity between the concentration of the object gas (NO) and the measured amount of light is poor. If the end of the inner tube 52 is further withdrawn from the end of the outer tube 51 (i.e., if the distance L2 is increased) to make a larger mixing space 55 and thus to obtain a better mixture, the site of reaction (i.e., the site of generation of light) becomes farther from the photometer 57, which also lowers the measurement efficiency.
Another problem is that, if impurity gas such as ammonia (NH.sub.3) or chlorine (Cl.sub.2) is included in the sample gas, the reaction with ozone produces precipitation deposit on the transparent glass plate 56 and obscures it.